The present invention relates to a method and a device for the production of foams using as an expanding agent carbon dioxide dissolved under pressure, whereby the substance to be expanded is mixed under pressure with preferably liquid carbon dioxide and subsequently expanded, with the formation of foam. The expandable substances used are in particular liquid starting products for plastics, which cure to form foamed plastics due to a polyaddition reaction or polycondensation reaction which commences after the expansion. The invention relates specifically to polyurethane foams.
In the production of polyurethane foams, a liquid or gaseous expanding agent is added to at least one of the reactive components (polyisocyanate and compounds containing hydrogen atoms reactive with isocyanate groups, in particular polyols). The other component is then added with mixing and the mixture obtained is conveyed either batchwise into a mold or continuously on a conveyor belt, where the mixture foams and cures.
For the production of the foam a number of methods have found wide application in practice. Liquids (such as low-molecular chlorofluoro-carbons, methylene chloride, pentane and the like) which evaporate at low temperature are often used. These materials evaporate out of the liquid reaction mixture and form bubbles. It is also possible to force air into the reaction mixture or into one of the components (mechanical foam production).
Finally, in the case of polyurethane foams, water is often added to the polyol component as the expanding agent. After mixture with the isocyanate component, the water liberates carbon dioxide as the expanding gas by reacting with the isocyanate (chemical foam production).
Liquid carbon dioxide has already often been proposed as an expanding agent on the grounds of environmental acceptability and industrial hygiene and owing to the comparatively high solubility of liquid carbon dioxide in the polyol component. See, for example, British patent 803,771 and U.S. Pat. Nos. 3,184,419, 4,337,318 and 5,120,770. However, liquid carbon dioxide has not gained acceptance in practice, apparently due to the difficulties of producing uniform foams during the necessary expansion of the reaction mixture from pressures of between 10 and 20 bar. The problem is, on one hand, that the carbon dioxide vaporizes relatively suddenly so that a very large increase in volume takes place in the reaction mixture, for example, by a factor of approximately 10, which is difficult to control. On the other hand, the reaction mixture tends to inhibit release of the carbon dioxide, which can be from 3 to 6 bar below the equilibrium vapor pressure of CO.sub.2 at the relevant temperature, so that a sudden explosive release of carbon dioxide occurs, with the result that large voids or bubbles are enclosed within the foam.
According to company literature from the Cannon Group, the problem of expansion by means of liquid carbon dioxide can be overcome by adding oxygen to the reaction mixture to form bubble nuclei, by carrying out the expansion in stages and by creating a special device for depositing the foam. Details of the method have not as yet been published and are not known.